The measurement of a state variable of a gas can be, in accordance with the invention, a quantitative measurement signalling a quantity of a gas or a proportion of a particular component in a gas or a qualitative measurement enabling a gas or combination of gases to be identified, or a monitoring of a gas for a change in a state variable or some identification of a particular state variable. Any combination of the foregoing types of measurements may be involved as well. A "state variable" as that term is used herein is intended to mean any thermodynamic parameter of a gas mixture or concentrations of the mixture components or combinations thereof and, for the purposes of the present application, the measurement of a state variable may also be said to include the detection of certain gases or certain levels of gas concentrations in a gas mixture or other environment.
Apparatus utilizing a semiconductive gas sensor, a power supply network in which the semiconductive gas sensor is connected and a measurement-signal processor or pickup responsive to the semiconductive gas sensor are known in a variety of forms. They can be constructed in various constructions and can be used for different purposes as will be apparent, for example, from German Utility Models G 91 13 607.5, G 93 09 638.0 and G 93 09 640.2, with the particular configuration being established with the end result and purposes in mind. The evaluation of the measurement signals can be effected by measuring temperature or other state variables of the gas by a comparison of two different states of the semiconductor gas sensor and from these measurements gas conditions and development patterns can be ascertained and compared with stored patterns to signal a particular condition or parameter of interest. The different states of the semiconductive gas sensor can be cyclically generated at the semiconductive gas sensor or the evaluation of the state of semiconductive gas sensor may be cyclically determined as desired or required.
Comparison can be effected between time-course evaluations of the semiconductive gas sensor output based upon recorded or stored values or patterns thereof, for example, after filtering of the output signals from the sensor, e.g. after low-pass filtering.
The comparison pattern may be determined by a calibration measurement from the measurement side or from some other source. Typical of the patterns which can be utilized are those which relate in general to composition determinations and proportions of the main components, changes in proportions and the like. Reference may be had to DE 43 21 736.2 A1 in this regard.
The semiconductive gas sensor can have a cover layer which is influenced by the gas and covering its entire surface or only part thereof and is affected especially by diffusion of oxidizing or other gases in metal oxides (DE 43 21 736.2 A1).
The physics of the relationship of the various surface phenomenon of such a device is developed in Phys. Stat. Sol. (a) 49, 27 ff. (1978).
Commercially available semiconductive gas sensors based upon metal oxides such as SnO.sub.2, WO.sub.3, Fe.sub.2 O.sub.3 and In.sub.2 O.sub.3 are available as are those which are based upon organic compounds, for example, phthalocyanines or pyroles. The choice of the sensor used for a particular purpose will depend, of course, on the measurements to be made and the particular characteristics of the sensor.
In the conventional devices over which the invention represents an improvement, the semiconductive gas sensor has an insulating layer and a conductor structure on one side of this insulating layer and an active semiconductive layer thereon in which the two electrodes are embedded in spaced relationship. The conductor structure can form part of a heating circuit which enables the operating temperature of the semiconductive gas sensor to be set. It is known to operate such a sensor with a pulsed or nonpulsed operating voltage, obtaining pulsed or nonpulsed measurement voltages and thus to provide one or more pulse-voltage generators in the circuit arrangement.
Experience has shown that the earlier device is susceptible to long-term stability problems, especially with reference to the zero point of the sensor. Indeed, a high degree of variability and fluctuations in the zero point are found. The long-term stability does depend upon the fabrication techniques as well as upon the specific operating conditions and can be influenced by the choice of working temperature, by the repetition frequency of the temperature fluctuations and by the gas components and their concentration fluctuations. The sensitivity of the semiconductive gas sensor varies, e.g. as a result of changing morphology, as variations in concentrations of catalytically active components of the gas, by adsorption of catalytically active substances, by irreversible adsorption of gases which tend to form highly stable solid compounds, and the like. These effects can render a semiconductive gas sensor more or less unusable after even a brief operating period.
As a consequence, the improvement of known devices for the purposes described so that the semiconductive gas sensors are more reliable and more precise with greater long-term stability, has been highly desirable.
To avoid the quality falloff, it has hitherto been the practice to recalibrate the semiconductive gas sensors with a standardization or test gas after more or less short operating intervals. That, of course, is expensive.